Detection systems which include a plurality of sensor units detecting and alerting to conditions such as smoke, fire, gas, motion, etc. are numerous and well known in the art. Some systems, such as those described in U.S. Pat. No. 5,587,705 for MULTIPLE ALERT SMOKE DETECTORS to Morris et al and U.S. Pat. No. 5,386,209 for CLUSTER ALARM MONITORING SYSTEM to Thomas disclose the use of different audible signals in a detector system to alert personnel of a condition as well as using RF signals to communicate with other detectors in the network. However, these systems, like many other similar systems, often require A.C. powered base stations or A.C. coupled detectors to facilitate network operation. This is disadvantageous in that it results in increased costs and network complexity such as running cables, providing connected base stations, maintaining large AC power sources, etc.
Another detector system described in U.S. Pat. No. 4,734,680 for DETECTION SYSTEM WITH RANDOMIZED TRANSMISSION to Gehman et al. discloses an integrated network of different sensors for communicating different conditions to an A.C. powered base unit or supervisory unit by means of randomized RF transmissions in order to avoid clashing between multiple units when sending to the supervising unit. While Gehman discloses a randomizing scheme for signal transmission, the Gehman et al. invention is directed toward transmission times between battery-operated sensor units and continuously active, A.C. powered base units, which are very short (on the order of milliseconds). However, transmission times between battery-powered sensing units and other similar battery-powered units are orders of magnitude longer (on the order of 10-30 seconds) to accommodate battery-saving duty cycles of receivers on the units. Thus, Gehman's system of randomly delaying transmissions between a sensor and base unit in hope of avoiding a clash is ineffective in a battery operated detection system which does not employ an A.C. base unit.
Still another system is described in U.S. Pat. No. 4,363,031 WIRELESS ALARM SYSTEM to Reinowitz, which discloses a wireless, battery-operated alarm system that contains at least three units placed in various locations throughout a building to sound an alarm when any one of them detects smoke. The units transmit and receive RF signals among one another in order to communicate the alarm condition. While this prior art avoids the problems of other systems by eliminating A.C. powered base stations and employing a purely battery-operated network of portable devices, an additional problem is presented. Since there is no longer a "master controller" or A.C. base station, situations may arise wherein oscillation by the originating sensor between an alarm or non-alarm (i.e. "all clear") which often occurs early in a fire might create a system conflict. Oscillation back and forth between alarm and "all-clear" states can result in total chaos, particularly where repeating units are present. A transmission loop might occur, for example, if a sensor detects an initial puff of smoke (prior to commencement of larger, consistent puffs) and a signal is initiated in response thereto to a second or more units. If the sensor during a brief interval between puffs no longer detects the smoke, it might send an all-clear signal back to the next units. The second or next unit would send the signals (alarm or all-clear) back to the first, which would send it back again and so forth until the batteries run down. This causes oscillation between alarm and non-alarm states, thereby running down the batteries and reducing detector lifetime. Furthermore, intermittent activation and deactivation of alarms due to oscillation conditions is undesirable, in that it suggests to the public who rely on the system for prompt and accurate detection that the system is either malfunctioning or simply unreliable. Further, the problem remains as to how to effectively transmit, receive, and control RF signals of a relatively long duration among a plurality of battery-operated sensors while avoiding multiple conflicting signals without depleting the finite battery power of the devices.
In light of these and other problems associated with the prior art, it is desirable to have a network of battery-powered sensors operable without the need of a base station to effectively communicate over a wireless RF communication scheme the occurrence of an emergency condition so that personnel located remotely from the alarm condition may be notified. It is also desirable that the detector system be operable with a number of different types of sensing devices to indicate various alarm or emergency conditions and include an associated priority scheme for prioritizing various conditions to most effectively alert the users to such conditions.